The term 'burden of responsibility' can be summarized as the obligation to take on a task or duty. The word 'ONUS' is a four-letter synonym for this phrase, and it is derived from the Latin word 'ONUS, ' which means 'burden.' This phrase is often used in legal contexts, where it refers to the responsibility of one party to prove or disprove a claim. In other contexts, it can refer to the responsibility of an individual or group to fulfill a certain task or duty.